1. Field of the Invention
The present invention relates to an inkjet printing apparatus that uses ink to perform printing. More particularly, the present invention relates to a process that is intended for use with a nozzle having a small number of ejections and intended to suppress an increase in image density associated with ink concentration.
2. Description of the Related Art
In an inkjet print head provided with a plurality of nozzles that eject inks, the inks are ejected from a plurality of nozzles according to print data to thereby form an image on a print medium. In such a print head, ejection frequencies of the respective nozzles are different depending on the image to be printed, and in a nozzle having a low frequency, a volatile component of ink inside the ejection port may vaporize to concentrate the ink. If the ink inside the nozzle is concentrated, the color material concentration per ejection volume is also increased, and as a result, the density of the image expressed on the print medium is increased more than necessary.
For example, Japanese Patent Laid-Open No. H11-320864 (1999) discloses a method that, in an inkjet printing apparatus that superposes different inks to perform printing, predicts an increase in the value of ink concentration according to the number of times of the non-ejection of each ink, and on the basis of the increased value, corrects a signal value corresponding to the ink. By employing Japanese Patent Laid-Open No. H11-320864 (1999), even in the case where the number of times of ejection is small, and an increase in ink concentration occurs, image density substantially equivalent to the density of the image printed with ink having no concentration increase can be achieved.
Meanwhile, the configuration of Japanese Patent Laid-Open No. H11-320864 (1999) is based on the assumption that, even if any concentration increase occurs, if the nozzle performs a single ejection, ink having the increased concentration is discharged, and in the subsequent ejection, the ink having an ordinary concentration is discharged. However, according to examination by the present inventors, it is recognized that there is the case where actual ink concentration gradually proceeds from near the ejection port into a nozzle, and concentrated ink cannot be fully discharged by a single ejection. That is, in Japanese Patent Laid-Open No. H11-320864 (1999), even in the case where in order to discharge concentrated ink, ejection is required two or more times, concentration of the ink inside the nozzle is not taken into account. Accordingly, in Japanese Patent Laid-Open No. H11-320864 (1999), on the assumption that ink concentration inside the nozzle is overcome by the first ejection, and in the second or third ejection, the ink has ordinary concentration, a signal value corresponding to each ink is corrected, and therefore the output image may have higher density than usual to give rise to density unevenness.